Method of cleaning and descaling ferrous bodies



Patented Mar. 3, 1953 OFFICE METHOD OF CLEANING AND DESCALING FERROUS BODIES Charles B. Francis, Pittsburgh, Pa.

No Drawing. Application June 2.5., 1948, Serial No. 35,288

7 Claims.

This invention relates to a method for cleaning the surfaces of ferrous bodies, and more particularly to the removal of oxides and scale therefrom and a fused bath therefor. While the invention may be applied in the cleaning of the surface of any ferrous body, it is designed especially for the removal of oxides and scale from the surfaces of bodies made'of steel containing appreciable amounts of chromium with or without appreciable amounts of nickel and other iron-alloying elements, such as tungsten.

.As examples of the steels to which the invention may be effectively and most economically applied, there may be .mentioned the high chromium steels known as stainless, corrosionresistant, and heat-resistant steels, many types of which contain,,besides chromium, 7.5% to 25% nickel and other elements, such as molybdenum andtungsten in smaller proportions. The layer of oxide or scale formed in the hot working of these steels is tightly bonded to the surface of the metal beneath and hence diflicult to remove mechanically. This layer is resistant to attack by acids used in pickling. The removal of the scale, which .isa necessary step in the production of. any finished article from such steels, is very difiiculty by the older'methods known to the art. Examples of other'steels to which the invention may be applied with advantage include the class known as tool steels which may contain up to 20%chromium in addition to other alloying elements,:such as molybdenum up to 7% and tungsten up to 2.1

Several types of fused baths have been proposed and used commercially for descaling corrosion-resistant steel. Among them are: (1) A bath composedchiefly of sodium nitrate (about 60%) and sodium hydroxide (about 40%) to which a small proportion of an oxidizing agent, such as sodium or potassium permanganate, may or may not be added; (2) a bath composed chiefiy of sodium hydroxide (95% or more) and sodium hydride (2% to 5%); and (3) a bath composed mainly of sodium hydroxide (85% or more) and a minor amount of an oxidizing agent, such as sodium peroxide, nitrate or chlorate. Each of these baths has certain disadvantages. For example, the .first is slow-acting; the second, which is reducing in action, is not only slow, but costly and somewhat hazardous to operate; and the third is slow in action, more or less hazardous to operate, and somewhat ineffective when applied to certain types or grades of alloy steel. All=these baths, furthermore, have to be operated at 'temperatures labove .800" F.

The chief object of my invention, therefore, is to overcome the disadvantages of the baths formerly used and to provide an improved bath better adapted for use under all conditions in the descaling of all types of steel to which a fused bath is applicable.

In accordance with my invention, I employ a fused bath composed of an alkali metal nitrate, preferably sodium nitrate, with additions of small amounts of an alkali metal hydroxide and an alkali metal oxide containing at least two atoms of oxygen per molecule.

A sufiicient amount of this salt is placed in a pot or container, having a form and dimensions adapted to accommodate the articles to be cleaned, and being made preferably of low carbon steel clad inside with Inconel, an alloy of nickel (about chromium (11% to 15%), iron (10%), manganese (1%) silicon (05% maximum) and carbon (0.15% maximum). This alloy will resist the action of the chemicals used for long periods of time when operated at temperatures between 650 F. and 800 F. which are normal for the process. Otherwise, the pot and setting are constructed to permit even heating of the contents to these temperatures by any convenient method known in the art. For treating many articles, such as plates, sheets and tubes, handled in lifts composed. of several pieces, it is advisable to equip the pot with a screw-type stirrer by means of which the liquid may be forced to circulate through the lift, either through the space between adjacent pieces, if sheets or plates, or through the articles themselves, as when the lift is made up of pipes or tubing.

After the salt has been added to the pot, heat is applied until the salt fuses and the volume of the bath is adjusted so that it will not overflow from the not when the articles to be treated are immersed therein. Next, I add to the bath from 2% to 5% by weight. of the hydroxide of the alkali metal selected. After the hydroxide has melted and a few minutes before the lift of articles isto be immersedinthebath, I add less than 3% and preferably from 0.1% to 11% by weight of a higher oxide of the" alkali metal, i. e., containing at least two atoms of oxygen per molecule, such as the peroxide, according to the composition and character of the scale to be removed, 0.1% to 0.5% being usually sufficient for descaling all typesof the stainless, corrosion and heatresistant steels. Thus the nitrate comprises from 94% to 98% of the bath, without allowance for impurities.

Besides-the concentration of peroxide, another factor affecting the activity of the bath is temperature, but as the temperature is raised above 700 F., the peroxide is decomposed and, to a less extent, the nitrate also, the rate of decomposition increasing rapidly as the temperature rises above 800 F. Below 600 F. the peroxide is not soluble in the bath and its solution is slow up to 650 F. These conditions, therefore, define and limit the working temperature of the bath to the range from 600 to 800 F.

To attain maximum speed and efficiency so as to clean or descale at the lowest cost per ton of steel treated, I prefer to preheat each lift of articles approximately to the temperature of the bath before immersing them therein; Preheating is particularly desirable in treating a large thick article or a number of articles having a large cross-section.

Preferably, the temperature of the bath is adjusted to between 700 F. and 750 F. The lift of steel articles, preferably preheated to 700 F., is immersed in the melt for a period of two minutes or more, as may be necessary, usually not over seven minutes, and the stirrer is started, or the bath is otherwise caused to circulate about the articles composing the lift.

The time of immersion will diifer for each type of steel and the conditions of heating to which its surfaces have been exposed. In any case, when action of the bath upon the scale has ceased, as determined by inspection or by experimental runs upon each type of scale to be removed, the lift is raised out of and held suspended over the bath for one or two minutes to permit as much of the adherent liquid as possible to drain 01f and flow back into the bath. The lift is then transferred to a tank of warm water. The articles are left immersed therein from one to two minutes to dissolve the film of salts adhering to the surface. Usually, the same tank of water is used repeatedly until the water becomes saturated with the salt. Therefore, after the articles have been immersed in the first tank, they are rinsed either by dipping in a tank of fresh water or spraying with water to remove the last of the bath salts and other products of the bath reactions.

, The rinse leaves the articles clean except for a reddish brown film of ferric hydroxide adhering to their surfaces. This may be removed by pickling, in which the articles are immersed in a hot bath of dilute sulpuric acid (5% to acid at about 180 F.) and subsequently rinsed or further treated according to the type of steel and the finish desired. In the case of articles made of stainless, or corrosion and heat resistant steels requiring a bright finish, the articles are removed from the sulphuric acid bath, sprayed or otherwise rinsed with hot water, and immersed in a bath of dilute nitric acid containing to 1% hydrofluoric acid, which brightens and passivates the surface, and are then rinsed with hot water and allowed to dry. To hasten this final operation, sheets and light plates are generally passed through a set of polishing rolls made of fabric discs.

As soon as one lift of steel has been removed from the bath and treated with water to remove the adhering salts, it is followed by another. The higher oxide of the alkali is added as required to keep the bath sufiiciently active and the nitrate and hydroxide are added in the proportion of 25 to 50 parts of the former to 1 part of the latter to make up for the drag-out loss.

As these operations are common to the practice of cleaning with any fused bath, it is unnecessary to explain in detail the principles and action of the bath upon the scales formed upon different types of steel to which my method of cleaning is applicable. The principles and reactions involved are most briefly and clearly explained by example, since the alkali-metal com pounds act similarly and elements alloyed with iron to form the steels treated fall into three classes, viz., those such as silicon, the oxides of which as formed in the scale are acids; those, such as nickel, the oxides of which as formed in the scale are bases; and those, such as chromium and tungsten, the oxides of which as formed in the scale are bases which may be oxidized to acids.

With sodium as an example of the alkali, a representative bath,'that is, a nitrate-hydroxideperoxide bath that has been used for several days, is one containing from 2% to 5% sodium hydroxide, from 0.1% to 0.5% sodium peroxide,

and about 90% sodium nitrate and nitrite, thenitrite formed from the sodium nitrate charged initially amounting to between 1% and 15% according to the rate of feeding the peroxide. As representative examples of the steels and scales to be treated, I list the following:

1. Stainless and heat resistant steels, the prin-- cipal alloying element of which is chromium, containing besides the usual elements from 3% to 30% chromium and from 0.2% to 2% silicon.-

The scale formed on these steels will consist of the lower oxides of iron, FeO and F8304, chm-- mous oxide, CrO, chromic oxide, C1203, and silica, S102.

2. Corrosion and'heat resistant steels contain-- ing from 7 to 35% nickel and from 8% to 30% chromium as the chief alloying elements. The

scale formed. on these steels from exposure to- From the above list, it is plain that the active, descaling reagents of the bath must react with, the basic oxides, FeO, F6304, NiO, CrO, CrzO3,-

and W02, and the acid oxide, SiOz, to formnew compounds soluble either in the fused bath itself,- in water, or in the acid solutions used. In the bath of my invention, the active reagents are sodium nitrate, NaNOa, sodium hydroxide, NaOI-I, and sodium peroxide, NazOz. Of these, the sodium peroxide is capable of acting alone, andthe sodium nitrate and hydroxide of acting jointly with all the basic oxides, except nickel oxide, to form, first, acid oxides, and, from these, neutral soluble salts in accordance with the follow-- ing reactions:

With the exceptionlof reactions 1 and 2, all the products of these reactions are partly soluble the bath and very soluble in water. The reaction: of sodium ferrite with water is'given later. The

oxygen liberated in reaction" 2- or sodium peroxide will oxidize metallic nickel; which bondsthe scale to the steel, thus:

The sodium oxide formed in these reactions.

This meta-silicate is insoluble in the-bath salts, but is soluble in water.

The reaction of sodium nitrate and sodium hydroxide jointly with the same oxide is. illus-- trated: by the following reactions:

The nitrate may also oxidize metallic nickel, and the hydroxide react with silica, thus:

It will be noticed that sodium nitrite is one of the products formed in reactions 9 through 12 and it remains to be stated that sodium peroxide is capable of oxidizing the nitrite to nitrate, thus:

(14) NaNOz-i-NazOwNaNOs-l-NazO These reactions illustrate a feature of the bath of my invention which distinguishes it from prior practice, viz., that it is to a great extent self-regulating. Thus, if the operator fails to add peroxide at the proper time or in sufilcient amount, the bath will still exert a descaling action, avoiding the necessity for a re-run. Also, as illustrated by reaction 14, my bath will consume less peroxide. The peroxide decomposes whether steel is being cleaned or not, and if nitrite is present, the peroxide reacts with it to restore the oxidizing capacity of the bath without loss. The nitrate may also be decomposed, but this reaction takes place more slowly and at a higher temperature than the decomposition of the peroxide.

Concerning the sodium ferrite formed in reactions 1, 2 and 9, some of this salt sinks in the bath to form a sludge with some of the chromate after the bath becomes saturated with these salts, but a certain portion along with the nickel oxide remains at the surface when the lift is withdrawn from the bath. When the lift is immersed in water, the sodium nitrate, nitrite and chromates dissolve, but the sodium ferrite reacts with water, thus:

Some of the ferric oxide, Fezos and nickel oxide, NiO, remain as a thin film or coating on the surface, and are removed by immersion in warm dilute hydrochloric or sulphuric acid, the latter being preferable because it is the cheaper and forms insoluble compounds with the cheap base, calcium hydroxide.

In order to reduce decomposition of the nitrate and peroxide, I may make use of a modified bath which is effective at a lower temperature, 1. e., from 600 to 650 F. The modified bath comprises an euteotiomixture-ofan alkali metal nitrate and an alkali: metal hydroxide meltingat about 515 E5 with a small addition of an alkali metal higher oxide. Specifically, the modified bath is composed of from 70% to ofsod-ium nitrate and itrom-20 to 30% sodium hydroxide, to which from= 0.1 to 1%. of sodium peroxide is added; Experimentalrunshave proved that such a bath operated at 600. F..is-.

very active, removing: oxides and scale. in. from.

one-half toone-third the .time required: by baths: composed mainly. of sodium hydroxide; The. sodium peroxide added dissolves; readily in:the;- bath and acts immediately to oxidize nitrite;

formed to. nitrate.. V V The. modified: bath is.- prepared; the; manners already explained. The lift of articles: to; be. cleaned is: immersed in the bathof molten chemicalsfor' a period of from 2; to 7 minutes-,, depending upon the temperature and, weightor the;v articles, and the naturerof. the'oxide or; scale to be removed. Usually from 2 to, 5,. minutes. immersion after the temperature of the articles has been raised to that of the bath is sufficient. Preheating, as explained above, shortens the necessary time of immersion.

The lift of articles is then raised and held suspended over the bath for a minute or so to permit the adhering liquid to drain back into the pot, and then immersed in a tank of water to remove the soluble constituents of the adhering film. These include all the constituents of the bath, and the sodium chromate formed as a product of the reactions between the scale and bath constituents. Another product of these reactions is sodium ferrite, NazQFezOs, which reacts with water to form sodium hydroxide and ferric oxide, some of which remains on the surface of the article along with the nickel oxide.

To remove these oxides, the articles are next immersed in a dilute hot solution of sulphuric acid, usually a 5% to 10% solution at a temperature of F. to 190 F. The action of this acid leaves the surface dull from a thin film of smut. To remove this film and passivate the surface, the articles are next immersed in a dilute hot solution of nitric and hydrofluoric acids, so described above, which treatment leaves the surfaces bright and covered with an invisible film of oxide resistant to corrosion. After a final rinse in hot water to remove the adhering film of acid, the articles are either permitted to dry or are dried in any of several different ways conventional in the art.

While I have referred specifically to the use of sodium compounds only in the practice of the invention, because they are the cheapest and most abundant in the United States, it is possible to substitute the corresponding compounds of potassium to give baths that are equally effective in removing oxides when operated at temperatures below 662 F., the decomposition point of potassium nitrate, and one of the higher oxides of potassium may be substituted for sodium peroxide. Even other strong oxidizing agents, such as sodium or potassium permanganate or chlorate may be substituted for sodium peroxide, but the use of such compounds is not recommended, because they increase the amount of sludge formed in the pot, and it is thought that chlorides act to decrease the corrosion resistance of the surfaces treated.

Although I have disclosed only a preferred example and practice of my invention and a modification thereof, it will be understood that changes in the procedure described may be made without departing from the spirit of the invention or the scope of the appended claims.

I claim:

1. In a method of descaling stainless steel articles, the steps including immersing them in a fused bath composed almost entirely of a mixture of an alkali metal nitrate and an alkali metal hydroxide with an addition of from .1 to 3% of an alkali metal oxide containing at least two atoms of oxygen per molecule and effective as an oxidizing agent at temperatures below 800 F., and maintaining said bath at a temperature of from 600 to 800 F.

2. The method defined by claim 1 characterized by the ratio of nitrate to hydroxide being from 20:1 to 50:1.

3. The method defined by claim 1 characterized by the ratio of nitrate to hydroxide being from 2.5:1 to4z1.

4. The method defined by claim 1 characterized by continuing the immersion for from 2 to '7 minutes.

5. The method defined by claim 1 characterized by said nitrate comprising 90% or more of said bath.

6. The method defined by claim 1 characterized by said nitrate composing from to of said bath.

'7. The method defined by claim 1 characterized by said mixture being the eutectic melting at about 515 F.

CHARLES B. FRANCIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,244,526 MacKay June 3, 1941 2,395,694 Spence Feb. 26, 1946 2,458,661 Webster et al Jan. 11, 1949 FOREIGN PATENTS Number Country Date 466,661 Great Britain May 26, 1937 

1. IN A METHOD OF DESCALING STAINLESS STEEL ARTICLES, THE STEPS INCLUDING IMMERSING THEM IN A FUSED BATH COMPOSED ALMOST ENTIRELY OF A MIXTURE OF AN ALKALI METAL NITRATE AND AN ALKALI METAL HYDROXIDE WITH AN ADDITION OF FROM .1 TO 3% OF AN ALKALI METAL OXIDE CONTAINING AT LEAST TWO ATOMS OF OXYGEN PER MOLECULE AND EFFECTIVE AS AN OXIDIZING AGENT AT TEMPERATURES BELOW 800* F., AND MAINTAINING SAID BATH AT A TEMPERATURE OF FROM 600 TO 800* F. 